Overdriving LED Electric Circuits
Overdriving occurs when the LED electric circuits draw excessive current beyond the maximum continuous rating as specific on the LED lights datasheet. Although overdriving can increase luminous output, it can also lead to excessive heat dissipation, and dramatically decrease lifespan. LED light datasheets normally contain a pulse or peak current rating, which exceeds the maximum forward current rating. These special drive conditions do not correspond with a continuous drive current, but rather a pulsed current. Pulse driving will allow the LED junction to cool subsequent to each electrical pulse. Although the forward current exceeds maximum rating, the momentary cooling periods between pulses prevent excessive heat, and damage to the PN junction. One disadvantage associated with pulse driving is the requirement for more complex LED electric circuits. In addition, LED lights tend to loose efficiency at higher drive currents. For this reason, it is usually best to drive LED circuits at the typical rating, in custom LED lightings designs.
2 LED Circuit Pulse Driving
Applications utilizing LED pulse driving techniques commonly employ higher than typical drive currents. A technique known as multiplexing often allows LED electric circuits to contain non-redundant components. This can drastically reduce production costs as well as overall physical dimensions. The basic operation of a multiplexed 2 LED circuit will illuminate LED lights in groups, where only a single group illuminates at any given point in time. This systematically allows every LED cluster or group to share a single driver circuit. This is how multiplexing eliminates redundant LED electric circuits, to provide cost savings. However, a reduction in luminous output results from LED switching, since not all LED lights within the array are simultaneously illuminated. To compensate, LED pulse driving employs very short but intense electrical pulses. Most LED lamp datasheets indicate a pulse current between two to five times that of the typical forward drive current. This method of LED overdriving should not lead to degradation assuming proper thermal management. Datasheets typically state a recommended pulse width and duty cycle.
Blue LED Circuits
LED optical wavelength will vary with drive current. In general, as junction current increases, wavelength will also increase. In an extreme case, an overdriven 2 LED circuit with orange light emitting diodes could actually appear somewhat red. An overdriven yellow LED may appear orange. Overdriven white LEDs may tend to appear somewhat like blue LED circuits. The human eye is most sensitive to color shifting associated with amber colored LEDs. The actual color shift occurs with temperature increase, a direct result of increasing current. Therefore, overdriving is not necessarily the only cause of color shift. Changes in the environment, specifically in ambient temperature, will also cause the LED color shift to occur. To demonstrate this phenomenon, place an LED in the freezer for approximately ten to twenty minutes prior to operation. You should immediately notice a color shift, and as the frozen LED thaws, the color will slowly fade back to its original wavelength. Note that the cold LED may also operate more efficiently and should result in an increased luminous output.
Junction Temperature (Tj)
In some applications, driving LED lights closer to the maximum ratings can increase the luminous output without adding additional LEDs to the array. However, Lunar Accents Design does not recommend overdriving any LED array without considering proper thermal management. LEDs operate less efficient at higher drive currents, and dissipate more energy in the form of heat. It is always best to refer to life expectancy data provided by the LED manufacture. The LED manufacturer should provide the junction temperature (Tj) at which their lifespan data applies. It is important to consider how life data corresponds to various dice temperatures and environmental factors. The LED manufacturer should theoretically supply life expectancy data corresponding with various dice temperatures. Although LEDs within some applications do not exceed the maximum dice temperature, these applications may not always maintain the capability of operation at optimal or minimal die temperatures. Note that most manufactures may provide extrapolate life expectancy data. Although accuracy of life data can vary, extrapolated data is better than no data. Some LED manufactures still do not offer any form of life expectancy data.